The present invention relates to a method and apparatus for measuring revolution speed, which generates pulse signals at an interval in inverse proportion to the revolution speed of a rotary member and calculates the revolution speed of the rotary member from the spacing of the pulse signals thus generated.
Although it is known to calculate the revolution speed of the rotary member either from the number of the pulses within a unit time or from the pulse spacing, errors in the calculation of revolution speed of the rotary member become larger at low speed or at high speed. Therefore, methods are known which use both the number of pulses N within a unit operation period TM, and also the lapse time T between the lead pulse of a unit operation period and the lead pulse of the previous unit operation period and the lead pulse of the previous unit operation period. Accordingly, the revolution speed V is determined as follows. EQU V=2.pi.RN/ZT=.alpha.N/T
In this relationship, the radius of the rotary member is R, and the number of pulses generated per revolution is Z. Rather than using the time period between lead pulses, the time period between final pulses can also be used.
FIG. 4 is a chart showing a relationship between a unit operation period TM, a lapse time T, and a number of pulses N. Because a varying lapse time T (like T1 through T4) is used instead of the fixed operation period TM, operational accuracy is improved. However, when the revolution speed becomes extremely low, the number N of the pulses fed into the unit operation period TM becomes zero (N=0). It therefore becomes impossible to carry out the above determination of revolution speed V.
In order to solve this problem, systems have been proposed which estimate the revolution speed for periods where N=0 so that system operation is not hindered (for example, see Japanese Laid-Open Patent Application Tokkaisho No. 62-241755). The above system estimates the speed for N=0 periods from the revolution speed and the acceleration obtained for the previous period. A measured speed is only obtained in the next period when a pulse is finally detected. In the meantime, the acceleration of this period is estimated using the estimated speed for previous period and the measured speed of the previous period. Also, when the acceleration for the previous period is positive, it is assumed to be zero (0) at the estimation of the speed for N=0 period.
In the system disclosed above, the estimated value of the speed for N=0 periods is obtained using the speed estimated during the previous period. When two or more N=0 periods follow each other, estimation errors cumulatively add to each other. Accordingly, output of the system varies considerably from the actual speed.
Furthermore, the speed curve connecting the measured speed with the estimated speed may not be smooth. The determined acceleration in particular, because it is the derivative of speed, becomes erratic. Moreover, such prior art systems cannot operate during acceleration. Such systems are directed only toward determining deceleration.